It is the practice in the oil industry to store large quantities of oil in suitable containers. Over a period of time, the containers can corrode with the result that the oil stored therein seeps out and can reach the ground or surface water. This is a particular hazard when the oil containers are buried underground because the oil seepage can remain undetected for long periods of time unless steps are taken to detect the oil leakage at source. In order to monitor any oil leakage from storage tanks or pipelines, it is known to drill a number of monitoring wells in the ground around the storage container sufficiently deep to meet the ground water.
Devices for measuring an oil water interface so as to detect any oil floating on the water surface are known. Thus, U.S. Pat. No. 4,503,383 (Agar et al.) discloses a device for detecting an interface between two fluids of differing electrical properties, the device having an electrical circuit comprising transmitting means for transmitting electrical impulses to the fluids and a detector for detecting electrical impulses. An electrically conductive probe is immersed into the water so as to transmit the electrical impulses therethrough and to receive electrical impulses therefrom, whereby the received impulses vary as a function of the electrical impedance of the fluid. Specifically, air has a higher impedance than oil which in turn has a higher impedance than water. Consequently, at an interface between oil and water or between air and oil, the detector will detect a change in impedance corresponding to the presence of oil.
The variation in the received impulses may be due to a change in dielectric constant of the fluid seen by the detector, variation in heating effect caused by microwaves.
It has also been suggested to use devices of this kind for monitoring the water-air interface in a monitoring well so as to detect oil which seeps into the monitoring well. In such a device, the probe disclosed by Agar et al. is replaced by at least one ring antenna which floats on the surface of the water so as effectively to monitor the water-air interface. Immediately oil seeps on to the surface of the water, the impedance seen by the antenna varies, thereby providing immediate detection of the presence of oil on the water surface.
In practice, such a system is very difficult to implement. Monitoring wells have a typical diameter of 100 mm and typical depths of tens of meters. The devices discussed above include complicated and vulnerable electronics which are easily susceptible to damage when the device is lowered into the monitoring well.
Additionally, the accuracy of such a device depends on the ring antenna or antennas lying in contact with the water surface. If, as a result of the self-weight of the device, the device sinks slightly within the water, the antenna which originally contacted the surface of the water will now be immersed therein. Consequently, the device will be unable to detect any oil which subsequently seeps on to the water surface. In order to avoid this problem, it is known to maintain the antenna slightly above the water surface so that any slight tendency to sink does not cause the antenna to become immersed in the water. However, the gap between the antenna and the water must then be completely filled by any oil seeping on to the water surface before the oil can be detected. Thus, it is required to minimize the gap between the antenna and the water surface without allowing the antenna to be immersed in the water. This requirement is extremely difficult to fulfil in practice.
Oil storage containers have also been proposed which consist of a storage tank surrounded by an outer jacket. It is then possible to provide oil detection devices within the outer jacket so that any oil seepage from the storage tank itself will be detected by the detection devices. Such a proposal offers the advantages of lower costs and the ability to detect if water seeps in due to rupture of the outer jacket or if hydrocarbons leak from the storage tank. However, the oil industry is not a new industry and there are large numbers of underground oil storage containers currently in use which are not provided with such outer jackets and from which possible oil seepage must constantly be monitored. For such oil storage containers it is usual to employ monitoring wells. For monitoring oil seepage from such oil storage containers using a device of the kind described, it is important to be able to lower the device until it reaches the water surface, to monitor remotely the water-air interface and, periodically, to be able to withdraw the device without damaging it.
When used for monitoring the presence of oil on ground water in a monitoring well, the device is formed of a material and is so dimensioned that, in use, it floats with the antenna substantially touching the air-water interface. The seepage of oil on to the water surface is monitored remotely via an electrical cable coupled to the device and by means of which it can be lowered through the monitoring well until it reaches the water therein. Clearly, the weight of the cable adds to the self-weight of the device itself tending to push it further into the water. Since, in order to function, the first antenna must lie substantially on the water surface, the weight of the cable must be compensated for and this is difficult. This height, corresponding to the water table or ground water height is unpredictable and varies throughout the year.
Furthermore, foreign matter falling on to the device also tends to make it sink further into the water thereby reducing its effectiveness to monitor oil seepage on the water surface.